Travis Swicegood, after having been asked to do a review of the Zend_Log component of the Zend Framework, wrote up his report and has also posted it to his blog.

The short answer to is Zend_Log good (enough): yes; the long answer, no. First off, it does what it needs to do without too much fuss [...] but... It has a few fatal flaws in my opinion.

While he found the simplicity of the component nice, the flaws seemed to outweigh the good. Problems like code redundancy and a simple thing like writing out to the log file isn't handled well (can't be accessed directly, only through the interface - slower in the long run?). He's included some tests to check out the difference.